User blog comment:Devilmaycry01/Devil May Cry still lives!/@comment-190.79.23.253-20150110080114/@comment-174.3.181.87-20150124214246

It's just that since Vergil was also an adept spellcaster, I can imagine him figuring out a spell that can allow him to transfer his power to others especially other demons or half-demons.

Perhaps Vergil gave Nero even more power using such a spell, and that was one of the last actions he did before he died?